An ELISA test developed in our laboratory was modified to optimize the system for determining the potency of inactivated polio vaccines (IPV) for types 1, 2, and 3. The optimum conditions of the reagents including pH, storage, incubation times, dilutions, and the concentrations of reagents were determined. Once standardized, we determined the reproducibility of the test by performing the ELISA test 5 times, for each polio type. Included in each test was the reference run as an unknown, and two vaccines run as the concentrate, final bulk, and final container, seven samples total. The samples were run in duplicate. We found our test to have excellent reproducibility, with the mean coefficient of variation being 1.2%, 2.7%, and 1.4%, for poliovirus types 1, 2, and 3, respectively. The 95% confidence upper and lower limits were calculated so that the potency of a vaccine tested by the ELISA could be judged with respect to the expected range, to either fall between the limits and pass or fall below the limits and be rejected. The limits were calculated for running the test 1, 2 or 3 times. Due to the excellent reproducibility of our ELISA test, the ranges of acceptibility of IPV products can be held rather narrow. This ELISA test will also be used as the gold standard to compare new reagents for use in the ELISA.